An NOx selective reduction catalyst (SCR) is in some cases used in an exhaust gas purifying apparatus for reducing nitrogen oxides (NOx) emitted from, for example, a diesel powered automobile, and an aqueous urea solution is used as its reductant. It is known that an aqueous urea solution with a urea concentration of 32.5 wt % can be advantageously used for effectively performing this reducing reaction. However, in the aqueous urea solution accommodated in a urea solution tank mounted in a diesel powered automobile, there are cases where the urea concentration changes due to such as a change over time. In addition, there is also a possibility of a different type of solution (such as light oil) or water becoming erroneously mixed into the urea solution tank. In view of such circumstances, urea sensors (urea concentration identifying devices) have been proposed to manage the urea concentration of the aqueous urea solution in the urea solution tank (e.g., refer to JP-A-2005-84026 (corresponding to US2007/00544091A1)).
The urea concentration identifying device in JP-A-2005-84026 (corresponding to US2007/00544091A1) is designed to provide an identifying device for a urea solution which is capable of accurately and speedily identifying the urea concentration of the urea solution even during the traveling of a vehicle. Namely, a concentration identifying sensor portion is provided with an indirectly heated concentration detecting portion and a liquid temperature detecting portion (detecting portion) each having a metallic fin. Further, this concentration identifying sensor portion (liquid concentration detecting element) is provided with a cover member for forming a urea solution introducing passage in such a way as to surround the metallic fins, as well as an enclosure with circulation holes respectively formed in upper and lower end face plates thereof.